variable resistor for mains >2kw rating

Lars

Hi,
I have a buffalo water boiler(40litre) with a 2.4kw element. I'm using it to brew beer. I want more control over the power output of the boiler so I can reduce the intensity of the boil and sometimes maintain lower temps, sometimes I'll want a good rolling boil and others I just need to maintain a temp lower than that. I've been thinking that the best way would be some sort of variable resistor in series with the heating element. I figure the element must have a resistance of about 24ohms (240^2/2400) so I was thinking that a 24ohm variable resistor rated for about 2.5kw or more would allow me to adjust the kettle output from about 600watts up to the full 2.4kw. Is there something like this that I can buy with these sort of ratings? alternatively I'd consider a bank of fixed resistors to give me maybe 600/1200/1800 watt outputs. Can anyone advise me where I might get this sort of stuff?

Thanks

L

concur4u?

you could look into some ac motor speed controlers ie various potentiometer/traic set up ie dimmer switchs etc or some rheostat devices

concur4u?

CR0026 - AC Motor Speed Controller (External-Signal/ Potentiometer) Module (230Vac, 2500W) something like this would suffice

Lars

Thanks Concur4u, but I was hoping for a simpler(and cheaper) solution. I've been looking at sites like digikey and I'm noticing that theres nothing rated anything like what I'm looking for.(youre probably aware of that, hence your recommendation). Is that type of speed controller my only option?

Thanks

L

concur4u?

off the top of my head that sort of wattage maybe some industrail fan speed controllers complete units but that one is pretty simple to mount in a plastic box with the dimmer button on the lid /cable out to boiler/ fused feed in and your good to go about an hour or less fitting but that sort of electronics would have to be used for your function

maybe something simpler look into burko boilers parts and functions as well
Burco Catering Urn Replacement Thermostat and Knob Kit maybe this wired into the circuit might also provide the function defo cheaper but more fitting etc

DublinDilbert

Why not fit a thermostat? Something designed for an immersion heater would definitely be able to switch that load.

An immersion controller will use on/off control, so will maintain the temp a few degrees either side.

Look in radionics too, they do lots of digital temperature controllers...

brightspark

As Dublin Dilbert said the best way would be to use a thermostat (not sure if an immersion stat would allow you to reach boiling point though)

Resistors would be impractical as they would have to be sufficiently large to carry the current. (I don't think even the old theatre dimmers would be large enough as they wouldn't normally have exceeded 2Kw)

A cheap option would be to use a simmer stat type control as commonly found on older electric cookers to control the heating rings

Using a simmerstat like this would be able to control the input power in a more efficient way than using resistors.

http://www.hawco.co.uk/Simmerstat-and-Knob_3260_p.aspx.


You might also be able to recycle one from an old cooker.

Remember you are working with mains voltages and large currents so make sure you do it safely.

Dardania

would it be possible to use a couple of immersion heaters, and just switch in as you need them i.e. one setting for 500W heat input, and another for the full 2kW?

liveandnetural

here what you want.......i just love the stat mounted in the lunch box
http://www.jimsbeerkit.co.uk/forum/viewtopic.php?t=617

Bruthal

Dardania wrote: »

would it be possible to use a couple of immersion heaters, and just switch in as you need them i.e. one setting for 500W heat input, and another for the full 2kW?

It is possible, even 1 dual 2kw/3kw immersion can be setup for 1.2kw, 2kw, 3kw and 5kw according to how its elements are switched in.

2 single 1kw elements would give 500w, 1kw and 2kw options.


But a variable stat would still be needed to properly control it anyway. But the lower wattage options coupled with the stat would give better control probably.

DublinDilbert

Just get a temperature controller and use it to switch a contactor:-

http://www.dealextreme.com/p/digital-temperature-controller-66125

Bruthal

DublinDilbert wrote: »

Just get a temperature controller and use it to switch a contactor:-

http://www.dealextreme.com/p/digital-temperature-controller-66125

That looks a good option alright. More accurate than a mechanical stat anyway.

Lars

Thanks guys, bought this on ebay, waiting for delivery...
http://www.ebay.co.uk/itm/3800W-Adjustable-0-220V-Voltage-Regulator-Dimming-Light-Lamps-Speed-Control-/290635791466?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item43ab3e3c6a


L

TheBadBadger

robbie7730 wrote: »

DublinDilbert wrote: »

Just get a temperature controller and use it to switch a contactor:-

http://www.dealextreme.com/p/digital-temperature-controller-66125

That looks a good option alright. More accurate than a mechanical stat anyway.

This is a practical solution lads but to maintain a certain temperature, this system seems like it will be constantly switching.. I. E. Once say 45 degrees is reached, contactor will release, only to pull back in almost immediately due to temp drop below 45

This type of system requires dead band control to operate efficiently.. And avoid constant switching..

On the other hand, I could be COMPLETELY overthinking this :-D

DublinDilbert

Yea all these on/off controllers will have hysteresis built in. You can usually set how many degrees it is, to minimise switching. So it will turn off @ 45c then not turn back on till 43c ect...

You can get controllers that use phase control which can vary the output from the controller. These typically have PI or PID control built in.

99% of the world uses on/off control and would probably be grand here.

Bruthal

Yea a standard stat wont be switching out at 45 degrees and back in at 44.99. There will be a decent hysterisis with any of them. Doesnt need to be that precise for this type of application.